The present invention relates to a particulate trap for trapping and removing particulates such as carbon and other noxious components contained in exhausts from a diesel engine.
Emissions from automotive engines are a leading cause of air pollution. It is therefore extremely important to develop a technique for removing noxious components contained in exhausts.
Exhausts from diesel engines are especially problematic because they contain mainly NOx and carbon particulates. Efforts are now being made to develop a technique for effectively removing such particulates from exhausts.
Trials have been made to remove noxious components from exhausts before exhausts leave the engine by Exhaust Gas Recycling (EGIR), or by improving fuel injectors. But none of these are decisive solutions. Now, it is considered more practical and effective to remove such noxious components from exhausts by means of a particulate trap after they leave the engine (as proposed in Unexamined Japanese Patent Publication 58-51235).
A particulate trap for trapping particulates contained in exhausts from a diesel engine has to satisfy the following requirements.
1 Particulate Trapping Efficiency
First, it is important that such a trap be capable of trapping particulates with high efficiency. Namely, such a trap has to be capable of trapping at least 60% of the particulates contained in exhausts from a diesel engine, though depending on their amount discharged and the load applied.
Of these particulates, small-diameter (2 .mu.m or less) suspended particulate matter (SPM) can reportedly trigger human lung cancers. Thus, it is especially important to trap such suspended particulate matter.
2 Pressure Drop
A second important requirement is that the trap can trap particulates with minimum pressure drop. The pressure drop when the exhausts pass through the trap tends to increase with the amount of particulates collected in the trap. If the pressure drop is too high, the engine performance may be hampered due to increased back pressure. Thus, it is considered necessary to keep the pressure drop below 30 Kpa. For this purpose, the particulate trap has to be capable of keeping the pressure drop at a sufficiently low level not only at the beginning of use but even after a large amount of particulates have been collected in the trap.
3 Regenerating Capability
A third requirement is that such a trap can be regenerated at a low energy cost. In order to use the particulate trap for a long period of time, it has to be regenerated periodically by burning trapped particulates. Heretofore, electric heaters and light oil burners were used to burn particulates.
4 Durability
The particulate trap has to be durable enough. Since it is exposed to high-temperature exhausts, it has to be highly corrosion-resistant. Also, it has to be capable of withstanding repeated heat shocks when burning particulates to regenerate.
5 Combininability with a Catalytic Converter
Some of today's cars have a catalytic converter carrying a catalyst for removing noxious gas components and mounted in an exhaust line of the engine. In such a case, both a particulate trap and a catalytic converter have to be mounted in the exhaust line. If they are separate members, they would require a larger installation space in the exhaust line, which is usually not available, as well as a higher installation cost. Thus, it is desirable to combine these member into an integral one-piece unit.
Among conventional filter elements of the above-described type, a wall-flow type, honeycomb-like porous filter element made of cordierite ceramic has been considered nearest-to-practical filters. But this filter element has some problems. One problem is that particulates tend to collect locally. Another problem is that due to its low heat conductivity, heat spots tend to develop during regeneration, so that the filter tends to melt and be damaged, or cracked due to thermal stress. Its durability is thus insufficient. Recently, a ceramic fiber trap formed by bundling ceramic fibers into a candle shape attracted much attention. This trap has, however, a problem in that its fibers tend to be destroyed due to decreased strength when exposed to high-temperature exhausts. Its durability is thus not sufficiently high either.
A metallic trap is now considered promising as a reliable, practical particulate trap because it is free of cracks during regeneration. But this trap cannot satisfy the above requirements 1 and 2, though it meets the requirements 3 and 4. Namely, if the holes of the filter are made small in an attempt to increase the particulate trapping efficiency, particulates tend to be trapped only on the surface of the filter, so that it will soon get clogged with particulates, increasing the pressure drop and shortening the life of the filter.
A wall-flow type, honeycomb-like porous member made of cordierite ceramic, which has been developed for use as a diesel particulate filter, has the following problems.
(1) Because of its low thermal conductivity, the filter cannot be heated uniformly during regeneration. Rather, heat spots tend to develop in the filter during regeneration. Due to such heat spots, the filter may partially melt, or develop cracks due to thermal stress. Its durability is thus unsatisfactory. PA1 (2) Since the inlet openings of the filter have a square section, heat produced by burning particulates during regeneration cannot dissipate efficiently. Rather, heat tends to concentrate on the corners of the square section. The filter is thus easily damaged or destroyed. Namely, its durability is poor. PA1 (3) This honeycomb-like porous member has many square openings whose inlet and outlet ends are alternately plugged. Such plugging increases the flow resistance, so that the pressure drop is high at an initial stage. PA1 (4) The exhaust inlet opening is so small in area that it can be easily clogged with particulates. Thus, the pressure drop tends to increase markedly even while the amount of particulates trapped is still small. PA1 (5) In order to trap suspended particulate matter (SPN) having particle diameters not exceeding 2 .mu.m, the diameter of the filter holes have to be made small. Such small filter holes will, however, make the pressure drop even higher. PA1 (6) If the filter carries a catalyst, high heat capacity of the honeycomb porous member may make it difficult to heat the catalyst quickly to a sufficiently high temperature to activate it. PA1 (1) A filter element comprising a plurality of tapered tubular filter members made of a nonwoven web of heat-resistant metal fiber and each having a small-diameter end and a large-diameter end. The filter members are concentrically and alternately inversely nested one inside another. The innermost filter member has its small-diameter end closed, while the gaps between the adjacent filter members are closed alternately at an exhaust inlet end and an exhaust outlet end by connecting the small-diameter end of each filter member to the large-diameter end of the immediately inner filter member. PA1 (2) A filter element comprising a sheet of filter material which is bent alternately inversely to close the gaps between the flat plate-shaped filter portions alternately at the exhaust inlet end and exhaust outlet end.
In contrast, a metallic trap is sufficiently high in thermal conductivity, so that crack-causing heat spots are difficult to develop during regeneration. Namely, such a metallic trap can be regenerated without the possibility of reduced durability. But if the filter is designed such that it can trap particulates with sufficiently high efficiency, particulates tend to be trapped only on the surface of the filter, so that it will soon be clogged with particulates. This leads to the shortening of filter life. Namely, conventional metallic traps cannot satisfy the second requirement.
A metallic trap that satisfies the second requirement would be one whose filter elements have a sufficiently large surface area through which exhausts pass (filtering area). But in order to increase the surface area (filtering area) of the filter elements in a conventional metallic trap, the size of the entire trap would have to be substantially increased. By welding filters to side plates with high accuracy, it will be possible to reduce the gaps between the filters and thus save the mounting space of the entire trap. But such accurate welding will lead to lower mass-productivity and higher manufacturing cost.
The particulate trap according to the present invention is made of a metal, so that it has none of the abovementioned problems of wall-flow type, honeycomb-like cordierite ceramic porous traps. Also, it is compact and nevertheless the surface area of its filter element is large, so that it has none of the problems of conventional metal traps, either. Alumina whiskers are provided on the surface of the filter material to catch suspended particulate matter (SPN). By carrying a catalyst on this metal trap, this device can be used both as particulate trap and a catalytic converter.